Let me just say... when you are FIRST learning to ride, everyone tells you to keep those heels down and so we PUSH them down and we don't have enough weight in the seat.
What I have found to experience is that over time you develop this beautiful thing called muscle memory and you do it naturally without having to "push" to "brace."
Mildot is correct that a relaxed leg will help you sit the trot better and that pushing or leaning against the stirrups isn't but first you need to figure out how to put all of your weight into the seat and down your legs.
Erika is right because you do need to keep those heels down, but not to push them down like you're making a hole into the earth, or bracing against them so you start slamming or popping onto your horse's back.
It's a fine balance.
But how I'd better describe the "feel" of the sitting trot is that your seat bones are moving with the horse. Your legs are on the horse but not pressing. They are stretched down all the way to your feet, and you are lightly keeping pressure onto the ball of your foot.
Now, the main thing you need to focus on is relaxing your hips.. tight hips makes for a very bumpy unpleasant ride. Secondly, the horse needs to not be hollow.. that also makes for a very bumpy ride. Thirdly, you need to stretch down and push all the weight into your seat and down the inside of your legs while making sure that your legs don't curl up into fetal position.
Work on one thing at a time, for a little bit of time, each time you get on your horse.
Also, remember... posting first helps both you AND your horse warm up and get ready to sit the trot.
It's hard.. but once you get it, it's awesome! Believe me, my horse's soft trot used to feel like a rocket launcher.